Microfluidic device and method for manufacturing a microfluidic device

ABSTRACT

A method for manufacturing a microfluidic device includes providing a first substrate having a first surface and a second surface located opposite the first surface. An etching mask is produced on the first surface, the etching mask having an opening. A recess is produced by etching in the first surface in a region of the opening. An electrically conductive material is deposited on the etching mask and/or a layer covering the etching mask, and on a region of a bottom of the recess below the opening.

CROSS-REFERENCE TO PRIOR APPLICATIONS

Priority is claimed to European Patent Application No. 19 210 724.1,filed on Nov. 21, 2019, the entire disclosure of which is herebyincorporated by reference herein.

FIELD

The present invention relates to a microfluidic device, and to methodsfor manufacturing a microfluidic device.

BACKGROUND

Microfluidic devices are designed to accommodate and/or conduct fluids,in particular liquids, in or into structures that have, at leastpartially, dimensions in the micrometer range (such as from a fewmicrometers to several hundred micrometers). For example, microfluidicdevices in the form of so-called “lab-on-a-chip” systems are used formedical or, in general terms, biotechnology applications. Accordingly,such microfluidic devices are sometimes also referred to as microfluidicchips. It is furthermore known to use microfluidic devices in the formof so-called flow cells, for example for diagnostic purposes, andspecifically for DNA sequencing.

Such a microfluidic device usually comprises a first substrate, forexample made of glass, quartz or silicon, having a surface in which oneor more recesses are provided, the recess(es) forming one or morecavities and/or channels. Suitable chemicals, electrodes, etc. can bearranged in the cavities and/or channels according to the particularfunction of the microfluidic device.

The cavities and/or channels are usually closed toward the top by a“cover” in the form of a second substrate.

Within the scope of the present description, information such as “towardthe top,” “upper,” “on,” “toward the bottom,” “lower,” etc. merelyrepresents relative information that denotes spatial relationships ofdifferent components with respect to one another once a reference systemhas been established, but does not define an absolute orientation in thespace.

The termination of the cavities and/or channels by the second substratein particular has to be fluid-tight (at least in sections). In theapplication, for example, an analyte may then be pushed through thechannels, and the corresponding analysis can be carried out.

The first substrate and/or the second substrate may be provided in theform of a wafer (e.g., made of glass or silicon) or as part of such awafer. In this case, the two wafers can be glued together, for example,using so-called wafer bonding, thereby closing the channel toward thetop in the fluid conduction part by means of the closure part.

For example, during the so-called adhesive bonding, an areal connectionis created between two wafers (such as glass wafers) by means of a thinadhesive film. This form of joining technique is used in particular inthe case of components that must not be exposed to elevated temperaturesin subsequent process steps, e.g., because metals or biologicalfunctional structures are already present, which do not tolerate ahigher temperature budget.

As an alternative to adhesive bonding, it is furthermore known to attacha surface of the second substrate by way of direct bonding (or fusionbonding) to the surface of the second substrate. Such a method does notrequire any adhesive or other intermediate layers. Instead, theattachment is based on chemical bonds between the surfaces, such as vander Waals forces, hydrogen bonds or strong covalent bonds.

A prerequisite for this is that the surfaces involved are sufficientlyclean, flat and smooth. In particular, the surfaces must not include anyexcessively large particles or protrusions. In other words, the surfacesmust have an extremely low surface roughness, for example of less than0.5 nm, in order to enable direct bonding. Any particle having aparticle diameter of 0.5 nm or more may result in non-contact sites(also referred to as flaws or “voids”) between the surfaces involved,wherein a diameter of the non-contact sites is usually larger than theparticle diameter by approximately a factor of 10,000. For example, a100 nm particle having a particle diameter of 100 nm may result in avoid having a diameter of approximately 1 mm.

Due to the above-described high requirements with regard to theproperties of the substrate surfaces, the direct bonding of wafers, inparticular glass wafers, to structured surfaces at the bond interface isgenerally not possible. In such cases, alternative bonding techniques,such as the aforementioned adhesive bonding, are thus commonly employed,which are capable of burying the steps of the structures or otherwisepreventing voids from forming around the structures.

In microfluidic devices, however, electrically conductive structures inthe region of a surface of the first substrate are sometimes requiredfrom a functional perspective, for example as metallic electrodes. It isnonetheless desirable to enable direct bonding to the surface of asecond substrate.

A manufacturing method for an optical waveguide is known from US5,525,190 A where channels formed in a surface of a substrate areprovided with a reflective metal layer from the inside. In this case,the metal layer is first deposited across the entire surface and onchannel bottoms and walls. The deposited metal is subsequently groundoff the surface, so that it only remains in the channels and isstructured accordingly. However, such a procedure, if it were applied tothe creation of metallic structures in microfluidic devices, would notreadily result in a sufficiently smooth surface that would allow directbonding to another substrate. Therefore, complicated grinding and/orpolishing processes would possibly be necessary in order to create theprerequisites for direct bonding.

SUMMARY

In an embodiment, the present invention provides a method formanufacturing a microfluidic device. A first substrate having a firstsurface and a second surface located opposite the first surface isprovided. An etching mask is produced on the first surface, the etchingmask having an opening. A recess is produced by etching in the firstsurface in a region of the opening. An electrically conductive materialis deposited on the etching mask and/or a layer covering the etchingmask, and on a region of a bottom of the recess below the opening.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described in even greaterdetail below based on the exemplary figures. The present invention isnot limited to the exemplary embodiments. All features described and/orillustrated herein can be used alone or combined in differentcombinations in embodiments of the present invention. The features andadvantages of various embodiments of the present invention will becomeapparent by reading the following detailed description with reference tothe attached drawings which illustrate the following:

FIGS. 1A-H schematically and by way of example show different steps andintermediate stages in a method for manufacturing a microfluidic deviceaccording to one or more exemplary embodiments; and

FIGS. 2A-D schematically and by way of example show different steps andintermediate stages in a method for manufacturing a microfluidic deviceaccording to one or more further exemplary embodiments.

DETAILED DESCRIPTION

In an embodiment, the present invention provides a method formanufacturing a microfluidic device which is as cost-efficient aspossible and in which electrically conductive structures are arranged inthe region of a surface of a first substrate. In any case, the method isto be compatible, in principle, with a direct bonding of the surface toa second substrate, i.e., it should in particular be able to result in asufficiently smooth surface.

In another embodiment, the present invention provides a correspondingmicrofluidic device that can be manufactured by such a method.

According to a first embodiment, a method for manufacturing amicrofluidic device is provided. The method initially comprises thefollowing steps: providing a first substrate having a first surface anda second surface located opposite the first surface; producing anetching mask on the first surface, the etching mask having an opening;creating, by means of etching, a recess in the first surface in theregion of the opening. The method furthermore comprises depositing anelectrically conductive material on the etching mask and/or a layercovering the etching mask, and on a region of a bottom of the recessbelow the opening.

An advantageous embodiment of the method comprises the following steps,before the deposition of the electrically conductive material: applyinga photoresist onto the etching mask and onto a region of a bottom of therecess below the opening; and removing the resist from the recess. Inthis embodiment, the electrically conductive material is deposited ontothe photoresist covering the etching mask, and onto a region of a bottomof the recess below the opening. The electrically conductive materialcan be deposited, in particular, onto the region of the bottom of therecess from which the photoresist was removed in the previous methodstep.

The etching mask is preferably undercut at least partially laterallywhen the recess is produced, so that at least one lateral edge sectionof the etching mask protrudes beyond an edge of the recess.

The first substrate is preferably present in the form of a wafer, forexample made of glass. The first surface may, for example, be a sectionof a front wafer surface (wafer front side). Accordingly, the secondsurface may be a section of a rear-side wafer surface (wafer rear side).

According to a second embodiment, a microfluidic device which has beenmanufactured by means of a method according to an embodiment of thepresent invention is provided.

According to a third embodiment, a microfluidic device comprises a firstsubstrate having a channel formed therein, wherein an electricallyconductive material is arranged in the channel, and wherein a bottom ofthe channel has lateral roundings in a cross section, in particular inthe form of etching radii. In the region of the roundings, the channelis free of the electrically conductive material.

For example, such a microfluidic device may have been manufactured by amethod according to an embodiment of the present invention.

Hereafter, reference will be made to all of the aforementionedembodiments.

Embodiments of the present invention achieve a microfluidic device whichis as cost-efficient as possible and in which electrically conductivestructures are arranged in the region of a surface of a first substrateby etching one or more recesses, e.g. in the form of one or morechannels, into the first substrate in a manner matching the conductivestructures to be produced. The structured etching mask used in theprocess remains in place after the channel has been etched. As a result,the subsequent process steps for producing the electrically conductivestructures become self-aligning. In particular, subsequent coatingprocesses may be carried out over the entire surface of a wafer, withoutthe need to carry out a targeted adjustment with respect to the etchedrecess(es).

The provided self-aligning method is both robust and cost-effective. Afurther advantage of the provided method is that direct bonding is alsopossible in this way with metallic electrodes (recessed in recesses) atthe bond interface.

In a microfluidic device manufactured according to an embodiment of theinvention, the use of the self-aligning method may be manifested inparticular in the absence of metallic conductive material in the etchingradii of the isotropically etched recess(es).

According to one embodiment, the removal of the photoresist from therecess includes exposing the photoresist in the recess. In the process,an exposure can take place through the second surface, i.e., forexample, from the rear side of the wafer.

In the process, the first substrate is preferably transparent to lightthat is suitable for an exposure of the photoresist. This makes itpossible for the photoresist in the recess to be exposed through thesecond surface using flood exposure.

In contrast, it may be provided that the etching mask is not transparentto light that is used for the exposure of the photoresist. Thus, in thecase of flood exposure through the second surface, the photoresist isnot exposed on the etching mask.

In this way, the photoresist can be deliberately removed from the recessby exposure from the rear side, and by subsequent development, whereasthe photoresist remains in place on the front side of the etching mask.

According to one advantageous embodiment, during the deposition, a layerthickness of the electrically conductive material in the recess isselected so as to be smaller than a depth of the recess. Later, thisallows direct bonding of the first surface to the surface of a secondsubstrate, without the electrically conductive material being in theway.

According to a preferred embodiment, the recess is a channel having adepth in the range of 0.01 μm to 5.0 μm, preferably in the range of 0.1μpm to 1.0 μm, such as 0.5 μm.

Alternatively, or additionally, it may be provided that the recess is achannel having a width in the range of 1 μm to 5000 μm, preferably inthe range of 1 μm to 500 μm, such as 100 μm.

In an embodiment, the provided method further comprises removing theetching mask. Depending on the material of the etching mask, this can beaccomplished, for example, by etching. For example, if chromium is thematerial used for the etching mask, the etching mask may be removed bymeans of chromium etching.

It may furthermore be provided that the first surface has a surfaceroughness of less than 0.5 nm. In this way, reliable direct bonding ofthe first surface to a surface of a second substrate, for example in theform of a second wafer, becomes possible.

It is, namely, also within the scope of an embodiment of the inventionthat, in a further method step, a second substrate can be attached tothe first surface by means of wafer bonding, in particular directbonding.

In one embodiment, a surface of the second substrate may include arecess, wherein the surface of the second substrate is attached to thefirst surface in such a way that the recess of the first substrate andthe recess of the second substrate are located opposite one another. Forexample, when a recessed metal electrode is accommodated on the firstsubstrate, and a microfluidic channel is accommodated on the secondsubstrate, direct contact between a medium (fluid) in the microfluidicchannel and the metal electrode can be established.

Glass, quartz or silicon may be used as preferred materials for thefirst substrate and/or the second substrate. Glass wafers in particularare suitable for the above-described manufacturing variant of amicrofluidic device by means of wafer bonding.

A microfluidic device according to the third embodiment can also beproduced in particular by means of a method according to an embodimentof the present invention. Accordingly, the above and subsequentexplanations with respect to the method according to embodiments thereofcan be analogously applied to the microfluidic devices according to thesecond embodiment and third embodiment, and vice versa.

Further details and advantages of the invention will become apparentfrom the following description of some exemplary embodiments based onthe figures.

Initially, a first substrate 1 having a first surface 11 and a secondsurface 12 located opposite the first surface is provided (see FIG. 1A).

The first substrate 1 is preferably present in the form of a wafer madeof glass. In an alternative variant embodiment, however, the firstsubstrate 1 can also be a wafer made of silicon or quartz, for example.

Here, the first surface 11 may, for example, be a section of a frontwafer surface (wafer front side). Accordingly, the second surface 12 maybe a section of a rear-side wafer surface (wafer rear side).

A structured etching mask 2 having at least one opening 20 is thenproduced on the first surface 11. The etching mask 2 may be a hard mask,such as a hard mask made of chromium. The etching mask 2 may, forexample, be structured by means of photolithographic methods known perse. This is indicated in FIG. 1A by a structured layer made of resist 3a.

In the cross-sectional view shown in FIG. 1A, a corresponding structuredetching of the material of the etching mask 2, for example chromium, hasalready taken place, as a result of which the opening 20 was produced.

FIG. 1B illustrates how subsequently a recess 110 is created in thefirst surface 11 by means of etching in the region of the opening 20.Here, the material of the first substrate 1 (for example glass) isetched isotropically with an etchant, for example HF or KOH.

It is easily apparent in the schematic cross-sectional view according toFIG. 1B that the etching mask 2 is partially undercut laterally, so thatlateral edge sections 21 of the etching mask 2 protrude beyond an edgeof the recess 110. It also becomes clear from FIG. 1B that a bottom 1101of the recess 110 has lateral roundings 1102 in a cross-section. Thelateral roundings 1102 are etching radii that are formed during theisotropic etching.

The recess 110 thus produced may be a cavity or a channel in which, forexample, a metallic electrode is to be subsequently arranged. Forexample, the recess 110 may take on the form of a channel having a depthT of up to several hundred nanometers, such as in the range of 0.05 μmto 5.0 μm, preferably in the range of 0.1 μm to 1.0 μm, such as 0.5 μm.A width B of the channel 110 may be in the range of 1 μm to 5000 μm, forexample, and preferably in the range of 1 μm to 500 μm, such as 100 μm.

After the etching process, the etching mask 2 is initially not removed,but rather is left in place together with the edge sections 21protruding laterally beyond the recess 110. In this way, the subsequentprocess steps for producing an electrically conductive structure 4 inthe recess 110 are self-aligning, so that, for example, some processescan be carried out on the entire wafer 1 without a targeted alignmenthaving to be carried out with the previously etched channel 110 (or, ifpreset, with a plurality of such channels 110).

This is to be clarified hereafter based on the further figures.

In a subsequent step, the wafer surface 1 may, for example, be coatedagain with a photoresist 3, 3 b, for example by means of spin coating.As a result of this step, the photoresist 3, 3 b covers the etching mask2 as well as a portion of a bottom 1101 of the recess 110 below theopening 20, as shown in FIG. 1C. In contrast, the lateral etching radii1102 below the protruding edge sections 21 of the etching mask 2 remainfree of photoresist 3, 3 b.

FIG. 1D shows another step in which the photoresist 3 b is removed fromthe recess 110. This is accomplished in a self-aligning manner by meansof an exposure and a subsequent development of the photoresist 3 b inthe recess 110. In the process, the (flood) exposure takes place throughthe second surface 12, i.e., from the rear side. This is indicated inFIG. 1D by vertical arrows L that point upwardly.

Since the material of the first substrate 1 (e.g., glass) is transparentto the light L used for exposing the photoresist 3b, while the materialof the etching mask 2 (e.g., chromium) is not transparent to this lightL, the photoresist 3 b is deliberately removed from the recess 110 in aself-aligning manner. The exposed section of the photoresist 3 b isshown hatched in FIG. 1D. However, the photoresist 3 remains in place onthe etching mask 2.

An electrically conductive material 4 is then deposited onto thephotoresist 3 covering the etching mask 2 and onto a region of a bottom1101 of the recess 110 below the opening 20, from which the photoresist3 b was previously removed. In this step, for example, a metallic layer4 (e.g., gold) may be deposited across the entire surface of the wafer1. This is schematically illustrated in FIG. 1E.

The deposition is preferably carried out in such a way that a resultinglayer thickness D of the electrically conductive material 4 in therecess 110 is no more than a depth T of the recess 110. This ensuresthat the electrically conductive material 4 does not protrude from therecess 110 beyond the original first surface 11, and that later directbonding, or other methods that require direct contact between the firstsurface 11 and another surface, are not impeded.

The lateral etching radii 1102 in the recess 110 are not coated duringthe deposition, i.e., they remain free of electrically conductivematerial 4.

In a further step, the result of which is illustrated in FIG. 1F, alift-off process removes the photoresist 3 (resist strip) along with theelectrically conductive material 4 arranged thereon, so that theelectrically conductive material 4 remains only in the recess 110.

The method furthermore comprises removing the etching mask 2, forexample by chromium etching, if the etching mask 2 is made of chromium.According to one variant, the photoresist 3 and the etching mask 2 canalso be removed in one step. As is shown in FIG. 1G, the metal 4recessed in the recess 110 as well as, surrounding the recess 110, thenow exposed first surface 11 of the first substrate 1 thus remain as aresult.

The first surface 11 is sufficiently clean and smooth for a subsequentdirect bonding. Thus, the first surface may have a surface roughness ofless than 0.5 nm, for example, after the removal of the etching mask 2.

FIG. 1H illustrates the result of a subsequent process step in which asecond substrate 5 is attached to the first surface 11 by means ofdirect bonding. The second substrate 5 closes the recess 110 toward thetop in the manner of a cover. FIG. 1H thus shows a section of across-sectional view of a microfluidic device 6 according to theinvention.

It may be provided in the process that the surface of the secondsubstrate 5 used during the direct bonding also has a recess, thissurface of the second substrate 5 being attached to the first surface 11during the direct bonding in such a way that the recess 110 of the firstsubstrate 1 and the recess of the second substrate 5 are locatedopposite one another. Thus, when the recessed metal electrode 4 isaccommodated on the first substrate 1, and a microfluidic channel isaccommodated on the second substrate 5, as shown, direct contact betweena medium (fluid) in the microfluidic channel and the metal electrode 4can be established.

FIGS. 2A-D schematically and by way of example illustrate a furthervariant embodiment of the provided method. The steps illustrated inFIGS. 2A-B are identical to those explained with respect to FIGS. 1A-B.In this regard, reference is made to what was said above. However, incontrast to the exemplary embodiment according to FIGS. 1A-H (inparticular FIGS. 1C-E there), the use of the photoresist 3 maysubsequently be dispensed with. The deposition of the electricallyconductive material 4, for example gold, may thus instead take placedirectly onto the etching mask 2 (for example made of chromium), asillustrated in FIG. 2C. The etching mask 2, together with the conductivematerial 4 deposited thereon, may then be removed by means of etching(FIG. 2D).

In this variant, the etching mask 2 itself (without photoresist)accordingly has the function of a “lift-off barrier” for theelectrically conductive material 4. For this purpose, it is advantageousif the layer thickness D of the electrically conductive material 4 isless than or equal to a layer thickness of the etching mask 2, and thusallows etching of the etching mask 2 when the electrically conductivematerial 4 has already been deposited thereon. Such a lift-off processis furthermore promoted if many channels 110 are present, which allowthe electrically conductive layer 4 to be undercut by an etchant, andthus the etching mask 2 to be attacked.

While embodiments of the invention have been illustrated and describedin detail in the drawings and foregoing description, such illustrationand description are to be considered illustrative or exemplary and notrestrictive. It will be understood that changes and modifications may bemade by those of ordinary skill within the scope of the followingclaims. In particular, the present invention covers further embodimentswith any combination of features from different embodiments describedabove and below. Additionally, statements made herein characterizing theinvention refer to an embodiment of the invention and not necessarilyall embodiments.

The terms used in the claims should be construed to have the broadestreasonable interpretation consistent with the foregoing description. Forexample, the use of the article “a” or “the” in introducing an elementshould not be interpreted as being exclusive of a plurality of elements.Likewise, the recitation of “or” should be interpreted as beinginclusive, such that the recitation of “A or B” is not exclusive of “Aand B,” unless it is clear from the context or the foregoing descriptionthat only one of A and B is intended. Further, the recitation of “atleast one of A, B and C” should be interpreted as one or more of a groupof elements consisting of A, B and C, and should not be interpreted asrequiring at least one of each of the listed elements A, B and C,regardless of whether A, B and C are related as categories or otherwise.Moreover, the recitation of “A, B and/or C” or “at least one of A, B orC” should be interpreted as including any singular entity from thelisted elements, e.g., A, any subset from the listed elements, e.g., Aand B, or the entire list of elements A, B and C.

LIST OF REFERENCE SIGNS

-   1 First substrate-   11 First surface-   110 Recess/channel-   1101 Bottom-   1102 Roundings-   12 Second surface-   2 Etching mask-   20 Opening-   21 Edge sections-   3, 3 a Photoresist-   3 b Exposed section-   4 Electrically conductive material-   5 Second substrate-   6 Microfluidic device-   B Width-   D Layer thickness of the electrically conductive material-   L Light-   T Depth

What is claimed is:
 1. A method for manufacturing a microfluidic device,the method comprising: providing a first substrate having a firstsurface and a second surface located opposite the first surface;producing an etching mask on the first surface, the etching mask havingan opening; producing, by etching, a recess in the first surface in aregion of the opening; and depositing an electrically conductivematerial on the etching mask and/or a layer covering the etching mask,and on a region of a bottom of the recess below the opening.
 2. Themethod according to claim 1, wherein the etching mask is undercut atleast partially laterally when the recess is produced such that at leastone lateral edge section of the etching mask protrudes beyond an edge ofthe recess.
 3. The method according to claim 1, further comprising,before the deposition of the electrically conductive material, thefollowing steps: applying a photoresist onto the etching mask and onto aregion of a bottom of the recess below the opening; removing thephotoresist from the recess; and then depositing the electricallyconductive material deposited onto the photoresist covering the etchingmask and onto the region of the bottom of the recess below the opening.4. The method according to claim 3, wherein the removal of thephotoresist from the recess comprises exposing the photoresist in therecess, the exposure taking place through the second surface.
 5. Themethod according to claim 3, wherein the first substrate is transparentto light suitable for an exposure of the photoresist.
 6. The methodaccording to claim 3, wherein the etching mask is not transparent tolight that is suitable for the exposure of the photoresist.
 7. Themethod according to claim 1, wherein the first substrate is made ofglass, quartz or silicon.
 8. The method according to claim 1, furthercomprising removing the etching mask.
 9. The method according to claim8, wherein the first surface has a surface roughness of less than 0.5 nmafter the removal of the etching mask.
 10. The method according to claim1, wherein a layer thickness of the electrically conductive material inthe recess is no more than a depth of the recess.
 11. The methodaccording to claim 1, wherein a second substrate is attached to thefirst surface by wafer bonding.
 12. The method according to claim1,wherein a surface of a second substrate has a recess and is attachedto the first surface such that the recess of the first substrate and therecess of the second substrate are located opposite one another.
 13. Amicrofluidic device manufactured by the method according to claim
 1. 14.The microfluidic device according to claim 13, wherein the firstsubstrate has a channel formed therein, a bottom of the channel havinglateral roundings in a cross-section, wherein the electricallyconductive material is arranged in the channel, and wherein the channelis free of the electrically conductive material in a region of theroundings.
 15. A microfluidic device, comprising: a first substratehaving a channel formed therein, a bottom of the channel having lateralroundings in a cross-section; and an electrically conductive materialarranged in the channel, wherein the channel is free of the electricallyconductive material in a region of the roundings.